This invention relates to metal tubing products, and more particularly, to metal tubing used in the automotive industry for applications such as brake lines, fuel lines and transmission oil cooling lines.
Tubing utilized in automotive applications requires corrosion and wear resistance that will allow it to last for the useful life of a vehicle. Also, the tubing must have abrasion resistance consistent with an automotive environment (i.e. stone impingement and chipping). Finally, the tubing should be able to isolate and absorb mechanical vibrations and acoustic noises. To satisfy these requirements, protective coating(s) are usually applied to metal tubing which is to be utilized in automotive applications.
Coatings used in the industry have generally been characterized by one or both of the following. First, a metallic substrate is deposited on the steel tube surface. Usually this is a sacrificial coating wherein the substrate corrodes before the metal tubing. Second, a barrier coating is deposited over the substrate to keep corrosive media from initiating corrosion and to provide increased abrasion resistance.
Examples of past materials and combinations of materials used as substrate and/or barrier layers in the automotive industry include: terne (an alloy of nominally 85% lead and 15% tin); zinc-rich paint over terne; a zinc-aluminum alloy (consisting of 95% zinc and 5% aluminum--available under the trademark GALFAN); aluminum rich paint over a GALFAN coating; electroplated zinc or zinc-nickel; PVF or PVDF over electroplated zinc; hot dip aluminum; epoxy and nylon.
These materials have been used as barrier and/or substrate layers in various combinations, but have experienced shortcomings that limit their usefulness. Prior art coating materials and methods have exhibited only limited resistance to wear and chipping from stone impingement and abrasion. Often, a shrinkable thermoplastic jacket is applied around conventionally coated tubes in order to provide improved chipping and wear resistance. Such methods, however, are very expensive and are not always effective. For example, shrinkable plastic jackets have only limited ability for absorbing or isolating mechanical vibrations and acoustic noises. Also, use of shrinkable plastic jackets is problematic in that the relatively high thickness of the jacket precludes its use under end fittings or connectors, thereby exposing the tube end to corrosion.
In order to overcome all of the problems (i.e. corrosion, wear, abrasion, chipping, stone-impingement, mechanical vibration, acoustic noise) encountered in automotive and fluid transport tubing applications simulaneously, specific polymer properties must be tailored for a tube coating. Since no single polymeric material is effective in combatting all problems, an effective product will take into account the relationship of polymer structures and properties as well as material processing and engineering application considerations.
Accordingly, the present invention provides a unique multi-layer polymer coating on metal tubing which manipulates the dynamic mechanical properties of polymeric materials to achieve protection against multiple elements for metal tubing used in automotive or fluid transport applications. It combines the unique dynamic mechanical properties of two layers of polymers to provide maximum effectiveness in corrosion resistance and wear, abrasion, chipping and stone impingement protection. Moreover, the multi-layer coating of the present invention is effective at absorbing impact energy and eliminating mechanical vibration and acoustic noises.